[C]Malloc 问题

Question

我正在尝试编写一个使用一些基本线程进行分配的程序。以下是我认为导致问题的相关 sn-ps。

程序在 25 个或更少线程时运行良好，但在使用 26 个或更多线程时通常会导致段错误。这使我认为我的 malloc 语句不正确。任何人都可以将我推向正确的方向吗？如果您需要发布更多代码，我很乐意提供。

而且，这些问题只有在我在学校的学生机器上运行程序时才会出现。它似乎在我的本地机器上运行良好。有人知道为什么吗？

感谢您的宝贵时间！

...

struct thread_args {
    struct bitmap *bm;
    double xmin;
    double xmax;
    double ymin;
    double ymax;
    int max;
    int start;
    int end;
};

...

int num_threads; //Given by user input
struct bitmap *bm = bitmap_create(500, 500); //All threads share the same bitmap
int i;
pthread_t *thread_id = malloc(num_threads * sizeof(*thread_id));
struct thread_args *args = malloc(num_threads * sizeof(*args));
for (i = 0; i < num_threads; i++) { 
    args[i].bm = bm;
    args[i].xmin = xcenter-scale;
    args[i].xmax = xcenter+scale;
    args[i].ymin = ycenter-scale;
    args[i].ymax = ycenter+scale;
    args[i].max = max;
    args[i].start = bitmap_height(bm) * i / num_threads;
    args[i].end = bitmap_height(bm) * (i + 1) / num_threads;

    pthread_create(&thread_id[i], NULL, compute_image, &args[i]);
}

for (i = 0; i < num_threads; i++) {
    pthread_join(thread_id[i], NULL);
}

...

void* compute_image(void *arg)
{
    struct thread_args* args = (struct thread_args*) arg;
    int i,j;
    int width = bitmap_width(args->bm);
    int height = bitmap_height(args->bm);

    // For every pixel in the image...

    for(j=args->start;j<args->end;j++) {
        for(i=0;i<width;i++) {
            // Determine the point in x,y space for that pixel.
            double x = args->xmin + i*(args->xmax-args->xmin)/width;
            double y = args->ymin + j*(args->ymax-args->ymin)/height;

            // Compute the iterations at that point.
            int iters = iterations_at_point(x,y,args->max);
            // Set the pixel in the bitmap.
            bitmap_set(args->bm,i,j,iters);
        }
    }
    return 0;
}

...

valgrind log

==24919== 
==24919== HEAP SUMMARY:
==24919==     in use at exit: 1,000,000 bytes in 1 blocks
==24919==   total heap usage: 56 allocs, 55 frees, 1,018,884 bytes allocated
==24919== 
==24919== Searching for pointers to 1 not-freed blocks
==24919== Checked 87,112 bytes
==24919== 
==24919== 1,000,000 bytes in 1 blocks are definitely lost in loss record 1 of 1
==24919==    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
==24919==    by 0x401256: bitmap_create (bitmap.c:21)
==24919==    by 0x400CEC: main (mandel.c:103)
==24919== 
==24919== LEAK SUMMARY:
==24919==    definitely lost: 1,000,000 bytes in 1 blocks
==24919==    indirectly lost: 0 bytes in 0 blocks
==24919==      possibly lost: 0 bytes in 0 blocks
==24919==    still reachable: 0 bytes in 0 blocks
==24919==         suppressed: 0 bytes in 0 blocks
==24919== 
==24919== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6)
--24919-- 
--24919-- used_suppression:      4 U1004-ARM-_dl_relocate_object
--24919-- used_suppression:      2 glibc-2.5.x-on-SUSE-10.2-(PPC)-2a
==24919== 
==24919== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6)

编辑：添加了 compute_image 代码和 valgrind 日志，尽管该日志缺少今天早些时候显示的错误消息。丢失的 1,000,000 字节是我所知道的。

Answer 1

确保pthread_create 没有返回错误。机器对可以生成的线程数有一个全局限制，并且可能会在接近该限制的情况下波动。如果您未能生成线程，您将得到一个垃圾pthread_t，这可能会导致pthread_join 爆炸。